1. Field of the Invention
The present invention relates to a fine particle dispersion liquid containing tantalum oxide fine particles, a tantalum oxide fine particle-resin composite, and a method of producing a fine particle dispersion liquid.
2. Description of the Related Art
In recent years, researches on the formation of a colorless, transparent, high-refractive index resin have been vigorously conducted with a view to increasing the refractive index of a resin. The formation involves introducing fine particles of a metal oxide having a high refractive index such as titanium oxide, zirconium oxide, zinc oxide, tantalum oxide, or niobium oxide, or a composite oxide of two or more of such metal oxides into the resin while maintaining a dispersed state.
The high-refractive index, transparent resin has been expected to find applications in various optical materials because the resin is a material bringing together the characteristics of an organic polymer such as transparency, flexibility, a light weight, and ease of molding, and the characteristics of an inorganic compound such as a high refractive index, a high strength, and heat resistance. Specific examples of the optical materials include: light guides provided for optical fibers, optical wiring boards, and the like; the parts and optical lenses of various instruments such as an image sensor, a camera, and a copying machine; various display materials; and resin compositions for sealing optical semiconductor devices such as a printed wiring board and a light-emitting diode.
It has been generally said that, in order for a metal oxide fine particle-resin composite to be transparent, the following requirements have only to be satisfied. That is, metal oxide fine particles are uniformly dispersed in a resin, and the volume-average particle diameter of each of the metal oxide fine particles is equal to or less than a quarter of the wavelength of visible light. It has also been generally said that the volume-average particle diameter is desired to be as small as possible. However, the metal oxide fine particles are apt to agglomerate as their particle diameters become smaller, and hence it has been difficult to disperse the metal oxide fine particles having small particle diameters in the resin uniformly and stably.
A method of solving the problem is, for example, an approach involving subjecting metal oxide fine particles such as zirconia particles to a dispersion treatment in the presence of a surface modifier such as a silane coupling agent to disperse the metal oxide fine particles in a solvent or a resin uniformly and stably as described in each of Japanese Patent Application Laid-Open No. H03-12460 and Japanese Patent Application Laid-Open No. 2008-120848.
As described above, a metal oxide having a high refractive index is, for example, titanium oxide, zirconium oxide, zinc oxide, tantalum oxide, or niobium oxide. Tantalum oxide has a wider band gap than that of any other high-refractive index metal oxide such as titanium oxide, zinc oxide, or niobium oxide, and hence is substantially free of photocatalytic activity with visible light. Accordingly, a tantalum oxide-resin composite has a small influence on a resin caused by the photocatalytic activity of tantalum oxide and is excellent in light fastness. Although zirconium oxide is also a metal oxide having a wide band gap and free of photocatalytic activity, the refractive index of zirconium oxide is lower than that of tantalum oxide. By reason of the foregoing, expectations have been placed on the potential of tantalum oxide to serve as a new high-refractive index material.
However, it has been difficult to obtain tantalum oxide particles each having high dispersibility in a solvent or a resin and a small crystallite size at an inexpensive price.